So I imagine this is a common question but I couldnt find any info other than the removal of the stock unit.
I am going to upgrade the head unit to something that has bluetooth and I notice all the speakers are routed to the amp via 4 pre-outs and my new"ish" head unit only has 2

That leaves me with a few options
#1 Buy new head unit with the proper # of pre-outs (would like a double din touchscreen for under $150) "recommendations welcome
#2 Could buy the little converter box (looks messy and confusing also heard rumors of pops/hissing noises coming from speakers)
#3 Use a pair of Rca-jack splitters so the amp receives signal on all 4 channels (has anyone tried this)

 

What is the make and model of this newish head unit? All the aftermarket units I have dealt with/looked at have the 4 channels.

When I change out a bose head unit, I use the PAC ROEM NIS2 line adapter. It works perfect, no pops, hisses or whatever.

If your car is a 2002 or 2003, there is a jumper you have to add for the instrument cluster.

 

yes it's 2003, what is the jumper for ? I see nothing on the dash that has anything to do with the audio system and i dont plan on connecting the steering wheel controls.
I also noticed upon removing the old head unit that it has 3 wiring dongles plugged into the back of it yet all the harnesses only have 2

 

The jumper is to make the trip odometers work. It is part of the many games car manufacturers play to make it difficult for a person to work on their own cars. You have to ground pin 22 in the biggest connector. I remember that Cadillacs would not start if the factory radio was removed. It was funny (to anyone that didn't own a Cadillac) because Cadillac had crap radios and the poor owners lost their cars for several days while the radio was repaired.

The Bose radio could have an optional 6 cd changer that was mounted in the trunk (I think). The 2 smaller connectors are for the radio power and speaker connections.

 

i believe the only 6CD changer was built into the HU itself, i think the mounted in the trunk is other makes, or aftermarket.

 

I ended up getting a Bluetooth FM transmitter. It's decent enough.
Gutting the BOSE system seemed like a huge project

 

it was worth it though ! i cant believe i didn't do it earlier, my car is modernized with many things from just a head unit upgrade. i can make hands free calling, and bluetooth is way better than FM and many other perks
my install was DIY and it wasn't too too bad.